Data storage devices are an essential element of any computer system. These devices have evolved to the point where enormous amounts of data may be stored on these devices and retrieved as needed.
FIG. 1 depicts the functional configuration of a conventional static storage device. The device 11 employs a mechanical head 13 that uses monochromatic radiation 15 to transfer static information from or to a location on a storage medium 17. To gain access to any given data, this mechanical head must traverse along a path defined with respect to the radius and length of the surface of the medium, seeking out the location of the recorded information desired. The time required for this mechanical device to traverse from one location on the storage medium to the next is referred to as “seek time”.
A signal is generated from static data off the surface of the storage medium 17 in linear, sequential fashion with the aid of the mechanical head 13. This signal is then transmitted for the purpose of being acted upon, manipulated by some means, or held in volatile memory.
Information is typically stored in data storage media as binary data. Binary data is typically represented by either a zero or a one, and is known as a bit. Data of this type may be either static or dynamic. Data which resides in a volatile state and which is being processed, transmitted, or otherwise acted upon, such as the data residing in Random Access Memory (RAM), is often referred to as dynamic data. By contrast, data which resides in a non-volatile state, such as the data residing on magnetic tape, magnetic disks, optical disks, and other such non-volatile media, is often referred to as static data.
Structured data, or information, is transmitted from one point to another as data signals. These binary data signals, which typically take the form of energy pulses, are generated for the purpose of storing, retrieving, processing, and transmitting information in the form of bits, bytes, words, packets, and the like. These signals (also called bit streams) are bit patterns that are structured sequentially, that is, structured linearly in one dimension. Hence, an energy pulse may be used to represent a bit of data within a bit stream that can be interpreted as logical lexicons such as “on or off”, “yes or no”, “0 or 1”, “true or false”, or any other type of discreet Boolean expression. Parallel bit streams are multiple sequential bit patterns that require independent channeling per bit stream. Nonetheless, the signal generated is structured linearly and in one dimension. For example, an eight-bit word is based on two discreet binary states to the power of three (23) The maximum number of unique combinations in such a word is sixty-four, and each of those sixty-four binary structures would be represented as some combination of these binary states in sequence (e.g., as 10110011). A simple method to represent this value would be to generate eight energy pulses with this sequence in a specified time. The signal is represented dimensionally as eight to the power of two, as this energy pulse is in either of two discrete Boolean states as noted above.
Conventional CD disks of the type presently available have about 3.52 inches of active area. In this area, there are about 64 thousand concentric circular tracks (about 16000 tracks per inch). The tracks in a conventional optical disk are similar to the grooves in a vinyl record in that a single long line contains all of the active information. Each track is about 0.6 micron wide, and the distance between tracks is about 1.6 micron. Data from the spiral track is in the form of depressions, called “pits”, and flat areas, called “lands”. To extract information from an optical disk, a laser is focused through a set of optics onto the tracks. The light reflected from the track will determine if the incident light has landed on a pit or a land. In particular, a pit will disperse the incident light almost completely, while a land will reflect light back. The incident light is passed through a one way mirror disposed at an angle to the incident beam so that light reflected from the track surface will be redirected towards a set of photodiodes for sensing and tracking.
Binary data passing within the area illuminated by the laser is accessed sequentially as the medium rotates, and a signal is subsequently generated which comprises logical, sequential bits that are to be interpreted. This reflected signal, containing the desired binary information, is collected linearly (that is, in one dimension). An electro-optic device mounted on the mechanical mechanism follows this track until the task of accessing the end of the desired recorded information is achieved, a process which can take several rotations of the medium to complete. Once the correct information has been located from the medium, conventional optical devices increase the rotational speed of the medium in order to access the data faster.
The time it takes to acquire the recorded static information from the surface of the medium is referred to as “access time”, and is a function of the rotational speed of the medium and of the electro-optics employed. When multiple requests to the same device occur, the time required for one process to complete before the next request can commence is known as the “lag time.”
Presently, the primary limitation in information retrieval speeds of conventional optical disk drives is seek time. This limitation is the principle reason why data transfer rates do not increase linearly as a function of disk rotation speed. In fact, tests have shown that the 24× optical disks currently available exhibit an improvement in operating performance of only about 20% when compared to 12× optical disks, rather than the approximately 100% improvement that might be expected if seek time were not a factor. The primary reason for slow seek times arises from conventional optical disk drive technology. While current optical disk drives are simple in design and construction, their performance is severely limited by the spatial distance the drive head has to cover, using a motor and gear mechanism, in order to access data located over several different tracks.
Another factor that reduces data retrieval speeds arises when multiple requests for data are sent to a single device. The submission of multiple requests has the effect of increasing the lag time and creating a bottleneck. Unfortunately, any decrease in data retrieval speeds can result in significant performance degradation in equipment which relies for its operation on the data retrieved from the data storage device. Typically, increasing the rotational speed of the data storage medium will not, by itself, compensate for increases in seek times and lag times.
Some attempts have been made to improve information retrieval speeds by constructing optical disk drives which utilize multiple mechanical devices or electro-optic heads to access multiple recorded informational areas. However, the additional cost in parts and electronic overhead makes this approach cost prohibitive for most applications. Other devices are provided with “look-ahead” algorithms to achieve some level of parallel accessing. However, the performance increases achievable with these devices are only incremental, and therefore do not adequately address the above noted problems.
There is thus a need in the art for methodologies for maximizing the performance and minimizing the seek time, access time, and lag time of optical disk drives and other memory devices. There is also a need in the art for memory devices which utilize such methodologies. These and other needs are met by the methodologies and devices disclosed herein and hereinafter described.